Chinese solar panel companies are receiving illegal subsidies on “massive” imports of their products in the United States, the Commerce Department’s International Trade Administration said on Monday in a preliminary determination.

Federal officials notified U.S. importers of solar panels and parts used to make solar panels from Wuxi Suntech Power Co., Ltd.; Changzhou Trina Solar Energy Co., Ltd.; and all other Chinese producers or exporters that a tariff could be imposed on imports retroactive to early December.

The preliminary determination is not enforceable but includes regulators’ initial findings that they have a “reasonable basis to believe or suspect” the allegations are valid. The determination states that the increase in Chinese-made solar panel imports has been “massive over a relatively short period of time,” based, in part, on an imports increase of 15 percent or more.

Industry officials in the United States urged caution in interpreting the findings before they are final.